Directly modulated lasers are susceptible to noise and distortions. Lasers that utilize amplitude modulation (AM) produce a light output that includes high frequency modulation (FM). The FM causes many unwanted system performance issues when light is transmitted over long optical fibers. This stems from the chromatic dispersion of optical fibers and from Rayleigh backscatter on optical fibers. The chromatic dispersion of fibers causes signals from an FM optical source to be dispersed in time as each signal component travels at a different speed corresponding to its momentary optical frequency which varies as the source is FM. Due to signal re-reflection in the fiber due to Rayleigh backscatter at density fluctuations in the fiber the optical signal at the output end of a fiber contains not only the intended signal but also time delayed versions of itself due to the re-reflection.
If the light source includes FM then these signals arrive at the detector, located at the end of the fiber, with different optical frequencies. Optical signals with different frequencies beat at the detector to produce RF signal frequencies equal to the difference of the frequencies in the optical domain. Thus unwanted RF signals are put out of the detector in addition to the wanted signal that give rise to noise and distortions.
Externally modulated transmitters may be utilized to modulate the light output of a laser. The transmitters may utilize AM to transmit the light output so the transmitted light includes very little FM. By using an externally modulated transmitter the problems noted above with directly modulated lasers are avoided because the laser itself is not modulated and therefore there is no FM generated in the laser. The modulator is driven by an RF signal. The modulator output is an AM version of the laser light. Depending on the modulator type this signal is free of FM or deliberately contains a small amount of FM that can be in-phase or counter-phase to the AM. Such small amounts of FM can in some cases be beneficial for system performance.
An externally modulated transmitter is expensive as it requires a separate laser and modulator. However an integrated laser/modulator chip is more cost effective. In such a device both a laser and a modulator section are integrated onto a single chip and if perfectly implemented the FM response is negligible. In practice however there are technological limitations that cause small reflections from the modulator section back into the laser. When the modulator is driven with a signal the magnitude of these reflections changes and this affects the laser operating condition such that the laser generates a small amount of FM. This problem is greater when the device is designed for high output power operation, such as in analog applications, because the laser output mirror (can be a grating) is designed to couple a large fraction of light out of the laser into the modulator section. Therefore the laser itself becomes more susceptible to light reflected from the modulator section in high power devices. High power devices however are important in analog applications. The resulting FM can be in-phase with the AM from the modulator or in counter phase, this depends on the nature of the reflections into the lasers and is difficult to control.
In an analog transmission system the resulting laser FM leads to noise and distortions at the receiver. Depending on device batch this noise can be negligible or severe and sorting of devices is needed to ensure adequate system performance. This negatively affects yield and production throughput.
In many transmission systems use is made of high-frequency modulation of the laser to obtain an FM output with a number of peaks in the optical spectrum. Thus, the power of the laser is spread over the peaks such that individual peak power is reduced. This limitation in individual peak power is useful to permit high launch power into optical fibers that would otherwise reflect this power due to SBS (Stimulated Brillouin Backscattering). The modulation of the laser is chosen such that the FM does spread the laser power over multiple peaks but does not detrimentally affect the noise or distortion performance of the transmission link. Similarly in some cases a low frequency modulation is added to the laser, also designed not to detrimentally affect the system performance.
FIG. 1 illustrates a block diagram of an example externally modulated optical transmitter 100. The externally modulated optical transmitter 100 includes a laser 110, a modulator 120, a modulation source 130, a pre-distortion circuit 140, and a modulator bias circuit 150. The laser 110 is to generate light and is operated at a bias point. The modulation source 130 is to add FM to the light generated by the laser 110 to suppress SBS and thus permit higher fiber launch power. The modulator 120 is to modulate the light from the laser 110 over a fiber 190 and is operated at a bias point. The pre-distortion circuit 140 is to generate uneven distortion orders that compensate for the S-curve (nearly symmetric around the bias point) of the modulator 120. The pre-distortion circuit 140 is driven by an input signal 160, such as a radio frequency (RF) signal, that may be initially provided to an amplifier 170. The modulator bias circuit 150 is to set the bias point of the modulator 120 at the center of the S-curve such that even order distortions are negligible. The output of the pre-distortion circuit 140 and the output of the modulator bias circuit 150 are combined, for example by a summer 180, and the resulting combination is provided as an input (e.g., control signal) to the modulator 120. The output of the modulator 120 (modulated light) is provided to the fiber 190.
By design the modulator 120 is preferably built to have negligible FM or a small amount of desired FM that can be in-phase or counter-phase to the AM of the modulator 120. However, in reality the modulator 120 may generate FM in response to the input signal 160 and the FM response (chirp) may be difficult to control. The externally modulated optical transmitter 100 does not have a means to reduce or adjust the chirp of the modulator 120 at the optical output. If the chirp of the modulator 120 is greater than desired, and the FM is included in the light transmitted over the fiber 190 it may result in noise and distortions and cause many unwanted system performance issues.